9)
So, when u /2, the modulator converts to the conventional model.
2.2 MODULATION SCHEME MIGRATION
In this section, we will present some commonly used modulation schemes. We will provide a migration
path for each modulation scheme plus an overall direction the wireless community is presently
taking. Whenever possible, we will compare the modulation schemes on the following merits:
Spectral efficiency (bps/Hz)
Receiver complexity (coherent or noncoherent detection)
BER performance
Transmit PA linearity requirements
We have chosen to begin with BPSK due to its simplicity and superior BER performance (see
Fig. 2.19). Here, spectral efficiency can be doubled when going to QPSK, with no sacrifice in BER
performance. If we turn our attention to spectral regrowth and receiver complexity, then two options
exist. If spectral regrowth is not an immediate concern, then the path on the right can be chosen to
select Differential QPSK (DQPSK), in order to have a simple receiver. Now, phase trajectories can
be manipulated to attempt to give an additional benefit of lower spectral regrowth through the use
of /4-QPSK and /4-FQPSK.
S(t) I(t) cos (vct) Q(t) cos (vct) cos (u) Q(t) sin (vct) sin (u)
S(t) I(t) cos (vct) Q(t) cos avct
p
2 b
S(t) I(t) cos (vct) Q(t) sin (vct)
62 CHAPTER TWO
PA +
X
X
+
X
cos(wct)
cos(wct)
??“sin(wct)
sin(q)
cos(q)
I
Q
S(t)
FIGURE 2.
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